Evolution of colour and phenolic compounds during Garnacha Tintorera grape raisining

The off-vine drying is one of the most important steps in the production of a high quality naturally sweet wine. However, only few studies have analyzed the changes in colour and phenolic compounds of the grapes throughout the process. In this work, UV/Vis spectrophotometry and HPLC/DAD-ESI/MS were applied to determine, respectively, the evolution of colour and phenolic compounds in the red grapes of Garnacha Tintorera during the raisining process. The total water loss in 83 days was about 62% and the sugar concentration rose from 225 to 464 g L−1. Browning and low-medium molecular weight compounds increase during the dehydration process. In CIELab coordinates, raisining decreases a∗ (red/green) and lightness and increases b∗ (yellow/blue), colour saturation and hue angle. In general, most of the phenolic compounds determined (anthocyanins, flavonols, esters of hydroxycinnamic acids, flavan-3-ol monomers and proanthocyanidins) increase but to a lesser extent than expected because of the water loss of the grapes during drying. The lower increase was observed for esters of hydroxicinnamic acids suggesting these compounds could undergo strong enzymatic degradation. Despite this, both the terminal and the extension subunit compositions show little changes, while the mean degree of polymerization of proanthocyanidins decreases as raisining progresses.